When a computer system is connected to a remote computer system via a public telephone network, a modem is typically used to connect the computer system to the telephone network. The function of the modem is to convert a digital signal into an analog signal that is suitable for transmission over the telephone network, and to convert an analog signal back into a digital signal.
FIG. 1 illustrates a prior art arrangement of connecting a host computer system 10 to a public telephone network 16 via a modem 11. As can be seen in FIG. 1, modem 11 typically includes a peripheral controller 17, a digital signal processor ("DSP") 12, a digital-to-analog ("D/A") and analog-to-digital ("A/D") converter 13, an isolation circuit 14, and a line interface circuit 15. DSP 12 is connected to computer system 10 via peripheral controller 17. D/A and A/D converter 13 is connected to DSP 12 and isolation circuit 14. Line interface circuit 15 is connected to isolation circuit 14 and external telephone network 16.
Line interface circuit 15 is used to interface with telephone network 16. Line interface circuit 15 typically includes the hardware for matching the line characteristics of telephone network 16. D/A and A/D converter 13 is used to convert digital signals into analog signals, and vice versa. DSP 12 is used to perform all digital signal processing functions of modem 11. DSP 12 typically performs the modulation and demodulation function of modem 11. Peripheral controller 17 is used to interface with host computer system 10 for transferring data to and from computer system 10.
Isolation circuit 14 is used to electrically isolate computer system 10 from telephone network 16. This is to satisfy the requirement of telephone network 16. Typically, a public telephone network is required to be electrically isolated from the user system. This is to prevent power surges occurred in either the user system or the public telephone network from affecting the other.
Disadvantages are, however, associated with this prior art arrangement. One disadvantage is that the isolation circuit used in the modem introduces noise and signal distortion to the signals transmitted to and received from the telephone network, thus reducing quality, accuracy, and data throughput of the data transmission between the host computer system and the telephone network. This is due to the fact that the isolation circuit is placed at the analog side of the modem. Another disadvantage is that the isolation circuit typically causes signal loss and reduces efficiency and accuracy in data transmission and reception.
As can be seen from FIG. 1, isolation circuit 14 is placed between converter 13 and line interface circuit 15 along the analog side of modem 11. Signals are transmitted along the analog side of modem 11 in analog form. Typically, isolation circuit 14 employs transformers or linear opto-couples in order to isolate computer system 10 from telephone network 16. As is known, the transformers and opto-couplers typically generate noise and signal distortion to the signals transmitted through isolation circuit 14. The noise and signal distortion greatly affects the signals transmitted when the signals transmitted are in analog form.